This invention relates generally to a current limiting device for use in electric power transmission and distribution systems and more particularly to an apparatus for triggering superconductors in a superconducting matrix fault current limiter.
In an existing superconducting matrix fault current limiter (MFCL) configuration (U.S. Pat. No. 6,664,875, assigned to the assignee of the present invention), a trigger matrix is connected in series with a current limiting matrix. The trigger matrix elements are used to provide necessary trigger magnetic fields to assist the transition from a superconducting state to a normal resistive state (the so-called “quenching” of a superconductor), of superconductor elements in the current limiting matrix. Using the first trigger matrix element (reference 310-1 in the patent) as an example, the magnetic fields are generated through current sharing between the superconductor trigger element RR1 and its parallel-connected trigger coils LL11 through LLm1. Since LL11 through LLm1 are physically wound around corresponding individual superconductor element R11 through Rm1 in the current limiting matrix, the magnetic field generated by the current in the trigger coils will cause R11 through Rm1 to quench. In this configuration, the development of current sharing in the trigger matrix is highly dependent on the timely and effective quenching of the superconductor trigger element RR1 during the initial rise of the fault current. Once the superconductor trigger element RR1 starts to quench after the fault current surges past its critical current level, a voltage develops across the element itself. This voltage then drives part of the surge current to the trigger coils to create a current-sharing regime.
In the MFCL, a parallel inductor coil LL1 is physically wound around RR1. The current in this coil after the current sharing develops will generate a magnetic field to assist further quenching of RR1. However, the initial quench of the RR1 relies solely on the fault current surging past its critical current level. Therefore, the trigger superconductor elements themselves need to be very reliable in order for the whole MFCL concept to work. These trigger elements should have no or minimal non-uniformity throughout the superconductor volume. The non-uniformity within the superconductor could causes hot spots generated due to uneven quenching and potentially damage the material. It is therefore desirable to have a mechanism, within the MFCL to drive the trigger superconductors to quench uniformly at its initial phase during current limiting, to ensure the successful development of trigger voltage across the trigger matrix and current sharing in the trigger coils.